Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. Such elements are described in, for example, Research Disclosure, June 1978, Item No. 17029 and U.S. Pat. Nos. 3,080,254, 3,457,075 and 3,933,508.
An important feature of the aforesaid thermally processable imaging elements is a protective overcoat layer. To be fully acceptable, a protective overcoat layer for such imaging elements should: (a) provide resistance to deformation of the layers of the element during thermal processing, (b) prevent or reduce loss of volatile components in the element during thermal processing, (c) reduce or prevent transfer of essential imaging components from one or more of the layers of the element into the overcoat layer during manufacture of the element or during storage of the element prior to imaging and thermal processing, (d) enable satisfactory adhesion of the overcoat to a contiguous layer of the element, and (e) be free from cracking and undesired marking, such as abrasion marking, during manufacture, storage, and processing of the element.
A particularly preferred overcoat for thermally processable imaging elements is an overcoat comprising poly(silicic acid) as described in U.S. Pat. No. 4,741,992, issued May 3, 1988. Advantageously, water-soluble hydroxyl-containing monomers or polymers are incorporated in the overcoat layer together with the poly(silicic acid). The combination of poly(silicic acid) and a water-soluble hydroxyl-containing monomer or polymer that is compatible with the poly(silicic acid) is also useful in a backing layer on the side of the support opposite to the imaging layer as described in U.S. Pat. No. 4,828,971, issued May 9, 1989.
One of the most difficult problems involved in the manufacture of thermally processable imaging elements is that the protective overcoat layer typically does not exhibit adequate adhesion to the imaging layer. The problem of achieving adequate adhesion is particularly aggravated by the fact that the imaging layer is typically hydrophobic while the overcoat layer is typically hydrophilic. One solution to this problem is that described in U.S. Pat. No. 4,886,739, issued Dec. 12, 1989, in which a polyalkoxysilane is added to the thermographic or photothermographic imaging composition and is hydrolyzed in situ to form an Si(OH).sub.4 moiety which has the ability to crosslink with binders present in the imaging layer and the overcoat layer. Another solution to the problem is that described in U.S. Pat. No. 4,942,115, issued Jul. 17, 1990, in which an adhesion-promoting layer, in particular a layer composed of an adhesion-promoting terpolymer, is interposed between the imaging layer and the overcoat layer.
The known solutions to the problem of providing adequate overcoat adhesion with thermally processable elements exhibit certain disadvantages which have hindered their commercial utilization. For example, while incorporation of a polyalkoxysilane in the imaging composition brings about a gradual increase in adhesion on aging of the element, the in situ hydrolysis of the polyalkoxysilane is slow and its rate is limited by the availability of water in the coated layer. Moreover, the alcohol which is formed as a by-product of the hydrolysis, for example, the ethyl alcohol that is formed by hydrolysis of tetraethoxysilane, is unable to escape through the highly impermeable overcoat layer and tends to migrate into the support. The support is typically a polyester, most usually poly(ethylene terephthalate), and migration of the alcohol into such a support causes a highly undesirable width-wise curl which makes the imaging element very difficult to handle. A serious consequence of such width-wise curl, even though it may be very slight in extent, is jamming of processing equipment.
The problem of unwanted curl can be reduced by use of the adhesion-promoting interlayer of U.S. Pat. No. 4,942,115, but this approach can result in adverse sensitometric effects and requires an additional coating step which makes it economically less attractive.
Unwanted curl can also be reduced by use of a barrier layer which is comprised of poly(silicic acid) and a water-soluble hydroxyl-containing monomer or polymer that is compatible therewith and which is interposed between the support and the image-forming layer, as described in the aforesaid copending commonly assigned U.S. patent application Ser. No. 020,912, filed Feb. 22, 1993, "Thermally Processable Imaging Element Comprising A Barrier Layer" by Wojciech M. Przezdziecki and Jean Z. DeRuyter. However, this method also requires the use of an additional coating step.
It is toward the objective of providing an improved method for the manufacture of thermally processable imaging elements which does not require an additional coating step and which effectively avoids the problem of width-wise curl that the present invention is directed.